The typical system to which the subject invention is directed is a rotational pointing control system comprised of a driver, a motor, and a mechanical plant. Specific examples would include control systems used for the precision pointing and movement of telescopes or scanning antennas.
In a typical satellite application, a motor and a plant to be driven are located at the far end of a flexible boom attached to a spacecraft. The motor which drives the platform has several intrinsic characteristics which make it very difficult to rotate the platform with constant acceleration. Such undesirable motor characteristics include internal friction, vibration, hysteresis caused by the frictional forces associated with ball bearings, and oscillations about the motor's shaft caused by a finite number of magnetic poles. Other components contribute to the problem with their own resonant disturbances because the motor is flexibly connected to the plant on one side, to the boom on the other side, the boom itself is flexible, and the other end of the boom is attached to a moving spacecraft.
Closed loop feedback systems are invariably employed in rotational pointing systems in order to accurately control movement and position because such a feedback system provides a means whereby any disturbance from a desired velocity or position may be sensed and corrected. In addition, closed loop feedback systems allow the control system to be designed without requiring that the engineer account for all of the minute characteristics of the specific plant to be driven.
While feedback control systems have been employed in the prior art to reduce the effect of disturbances, such standard feedback systems have never provided more than several microradians of accuracy when used with standard motors. The prior art feedback control systems are only capable of achieving sub-microradian pointing and moving accuracy when used with expensive magnetic suspension motors that do not exhibit many of the inherent disturbances associated with standard motors. However, using the prior feedback techniques with magnetic suspension motors is an undesirable solution for improving accuracy because magnetic motors are expensive and consume large amounts of power.
Although multiloop feedback systems have been employed in pointing control systems, such prior art control systems typically employ a common feedback loop around the entire system and only current feedback around the motor. The common feedback loop reduces the effects of variations in the mechanical plant parameters and of motor and gear train friction and cogging. However, the achievable loop gain in the common loop is limited by inherent disturbances associated with a flexible plant and high frequency sensor noise. These limitations are such that the achievable loop gain in the common loop is frequently insufficient to reduce the effects of motor friction and cogging.
Another type of feedback known in communication engineering is bridge feedback. In communication engineering, bridge feedback refers generally to the concept of feeding back both current and voltage in a feedback amplifier stage. Both single and double loop feedback systems are known to date in the communication field. However, double loop bridge feedback has not been applied to the problem of mechanical control systems.